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FORMULAS AND NOMENCLATURE OF IONIC AND COVALENT COMPOUNDS
Adapted from McMurry/Fay, section 2.10, p. 56-63 and the 1411 Lab Manual, p. 27-31.
TYPES OF COMPOUNDS
Ionic compounds are compounds composed of ions, charged particles that form when an atom
(or group of atoms) gains or loses electrons. (A cation is a positively charged ion; an anion is a
negatively charged ion.) Covalent or molecular compounds form when elements share
electrons in a covalent bond to form molecules. Molecular compounds are electrically neutral.
Metal + Nonmetal → ionic compound (usually)
Metal + Polyatomic ion → ionic compound (usually)
Nonmetal + Nonmetal → covalent compound (usually)
Hydrogen + Nonmetal → covalent compound (usually)
TYPES OF IONS
Main-Group Metals (Groups IA, IIA, and IIIA)
Group IA, IIA, and IIIA metals tend to form cations by losing all of their outermost (valence)
electrons. The charge on the cation is the same as the group number. The cation is given the
same name as the neutral metal atom.
Ions of Some Main-Group Metals (Groups IA – IIIA)
Group Element Cation Ion name Group Element Cation Ion name
IA H H+ hydrogen ion IIA Mg Mg2+ magnesium ion
+ 2+
Li Li lithium ion Ca Ca calcium ion
+ 2+
Na Na sodium ion Sr Sr strontium ion
K K+ potassium ion Ba Ba2+ barium ion
+ 3+
Cs Cs cesium ion IIIA Al Al aluminum ion
Transition (B-group) and Post-Transition (Group IVA and VA) Metals
These elements usually form ionic compounds; many of them can form more than one cation.
(The charges of the transition metals must be memorized; Group IV and V metal cations tend to
be either the group number, or the group number minus two.)
Many of these ions have common or trivial names (-ic endings go with the higher charge,
-ous endings go with the lower charge). The systematic names (also known as the Stock system)
for these ions are derived by naming the metal first, followed in parentheses by the charge written
in Roman numerals. For the metals below that typically form only one charge, it is not usually
necessary to specify the charge in the compound name.
The mercury I cation is a special case; it consists of two Hg+ ions joined together, and so
is always found as Hg 2+. (Hence, mercury(I) chloride is Hg Cl , while mercury (II) chloride is
2 2 2
HgCl .)
2
Ions of Some Transition Metals and Post-Transition Metals (Groups IVA and VA)
Metal Ion Systematic name Common name
2+
Cadmium Cd cadmium ion
2+
Chromium Cr chromium(II) ion chromous ion
3+
Cr chromium(III) ion chromic ion
2+
Cobalt Co cobalt(II) ion cobaltous ion
3+
Co cobalt(III) ion cobaltic ion
+
Copper Cu copper(I) ion cuprous ion
2+
Cu copper(II) ion cupric ion
Gold Au3+ gold(III) ion
Iron Fe2+ iron(II) ion ferrous ion
Fe3+ iron(III) ion ferric ion
Manganese Mn2+ manganese(II) ion manganous ion
Mn3+ manganese(III) ion manganic ion
Mercury Hg 2+ mercury(I) ion mercurous ion
2
2+
Hg mercury(II) ion mercuric ion
2+
Nickel Ni nickel(II) ion
+
Silver Ag silver ion
Zinc Zn2+ zinc ion
Tin Sn2+ tin(II) ion stannous ion
Sn4+ tin(IV) ion stannic ion
Lead Pb2+ lead(II) ion plumbous ion
Pb4+ lead(IV) ion plumbic ion
3+
Bismuth Bi bismuth(III) ion
5+
Bi bismuth(V) ion
Main-Group Nonmetals (Groups IVA, VA, VIA, and VIIA)
Group IVA, VA, VIA, and VIIA nonmetals tend to form anions by gaining enough electrons to
fill their valence shell with eight electrons. The charge on the anion is the group number minus
eight. The anion is named by taking the element stem name and adding the ending -ide.
Ions of Some Nonmetals (Groups IVA - VIIA)
Group Element Anion Ion name Group Element Anion Ion name
4– 2–
IVA C C carbide ion VIA Se Se selenide ion
4– 2–
Si Si silicide ion Te Te telluride ion
3– –
VA N N nitride ion VIIA F F fluoride ion
3– –
P P phosphide ion Cl Cl chloride ion
3– –
As As arsenide ion Br Br bromide ion
2– –
VIA O O oxide ion I I iodide ion
2– –
S S sulfide ion IA H H hydride ion
Polyatomic Ions
Polyatomic ions are ions that are composed of two or more atoms that are linked by covalent
bonds, but that still have a net deficiency or surplus of electrons, resulting in an overall charge on
the group. A metal plus a polyatomic ion yields an ionic compound.
Formulas and Names of Some Polyatomic Ions
+ 2–
NH4 ammonium CO3 carbonate
+ –
HO hydronium HCO hydrogen carbonate (bicarbonate)
3 3
– –
OH hydroxide OCN cyanate
– –
CN cyanide SCN thiocyanate
2- 2–
O peroxide S O thiosulfate
2 2 3
- 2–
N3 azide CrO4 chromate
– 2–
NO nitrite Cr O dichromate
2 2 7
– 2–
NO nitrate SO sulfate
3 4
– 2–
ClO hypochlorite SO sulfite
3
– –
ClO2 chlorite HSO4 hydrogen sulfate (bisulfate)
– 3–
ClO3 chlorate PO4 phosphate
– 2–
ClO4 perchlorate HPO4 monohydrogen phosphate
– –
MnO permanganate HPO dihydrogen phosphate
4 2 4
– - –
CHO acetate (OAc) HSO hydrogen sulfite (bisulfite)
2 3 2 3
2–
CO oxalate
2 4
There are some regularities in the names of these polyatomic ions.
a. Thio- implies replacing an oxygen with a sulfur:
2– 2–
SO = sulfate S O = thiosulfate
4 – 2 3–
OCN = cyanate SCN = thiocyanate
b. Replacing the first element with another element from the same group gives a polyatomic ion
with the same charge, and a similar name:
Group VIIA Group VIA Group VA Group IVA
– 2– 3– 2–
ClO chlorate SO sulfate PO phosphate CO carbonate
3– 4 2– 4 3– 32–
BrO bromate SeO selenate AsO arsenate SiO silicate
3 4 4 3
– 2–
IO3 iodate TeO4 tellurate
c. Some nonmetals form a series of polyatomic ions with oxygen (all having the same charge):
– – – –
ClO , hypochlorite; ClO2 , chlorite; ClO3 , chlorate; ClO4 , perchlorate. The general rule for
such series is:
y– 2–
XO stem + -ate SO sulfate
n 4
y– 2–
XO stem + -ite SO sulfite
n-1 3
y– 2–
XO hypo- + stem + -ite SO hyposulfite
n-2 2
y– 2–
XO per- + stem + -ate SO persulfate
y–n+1 2–5
X stem + -ide (the monatomic ion) S sulfide
Note that in some cases, the -ate form has three oxygens, and in some cases four oxygens.
(These forms must be memorized.)
Writing Formulas of Ionic Compounds
1. The positive ion is given first, followed by the monatomic or polyatomic anion.
2. The subscripts in the formula must produce an electrically neutral formula unit. (That is, the
total positive charge must equal the total negative charge.)
3. The subscripts should be the smallest set of whole numbers possible.
4. If there is only one of a polyatomic ion in the formula, do not place parentheses around it;
e.g., NaNO , not Na(NO ). If there is more than one of a polyatomic ion in the formula, put
3 3
the ion in parentheses, and place the subscript after the parentheses; e.g., Ca(OH) ,
2
Ba (PO ) , etc. [Remember the Prime Directive in writing formulas: Ca(OH) ≠ CaOH !]
3 4 2 2 2
+ -
Na Cl NaCl
Ca2+ Br- CaBr
+ 2- 2
Na S Na S
2+ 2- 2
Mg O MgO
Fe3+ O2- Fe O
+ 2- 2 3
Na SO Na SO
4- 2 4
Mg NO Mg(NO )
3 3 2
NH+ SO 2- (NH ) SO
4 4 4 2 4
Nomenclature of Ionic and Covalent Compounds
1. Binary Ionic Compounds Containing a Metal and a Nonmetal. A binary compound is a
compound formed from two different elements. There may or may not be more than one of
each element. A diatomic compound (or diatomic molecule) contains two atoms, which
may or may not be the same.
Cl Not binary (only one type of atom), but diatomic (two atoms).
2
BrCl Binary and diatomic. (Two atoms, and they’re different elements.)
HO Binary, since there are only two types of atoms.
2
CH Binary, since there are only two types of atoms.
4
CHCl Not binary or diatomic.
3
Metals combine with nonmetals to give ionic compounds. When naming binary ionic
compounds, name the cation first (specifying the charge, if necessary), then the nonmetal
anion (element stem + -ide). Do NOT use prefixes to indicate how many of each element is
present; this information is implied in the name of the compound.
NaCl Sodium chloride
AlBr Aluminum bromide
3
Ca P Calcium phosphide
3 2
SrI Strontium iodide
2
FeCl Iron(II) chloride or ferrous chloride
2
2. Ionic Compounds Containing a Metal and a Polyatomic Ion. Metals combine with
polyatomic ions to give ionic compounds. Name the cation first (specifying the charge, if
necessary), then the polyatomic ion as listed in the table above. Do NOT use prefixes to
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